1. Technical Field
This invention relates to audio systems and, more particularly, to speakerphone circuits which connect to an audio line for providing two-way voice communications.
2. Description of the Prior Art
The use of analog speakerphones have been the primary hands free means of communicating during a telephone conversation for a great number of years. This convenient service has been obtained at the price of some limitations, however. These speakerphone usually require careful and expensive calibration in order to operate in an acceptable manner. They are also designed to operate in a worst-case electrical and acoustic environment thereby sacrificing the improved performance that is possible in a better environment.
The operation of conventional analog speakerphones is well known and is described in an article by A. Busala, "Fundamental Considerations in the Design of a Voice-Switched Speakerphone," Bell System Technical Journal, Vol. 39, No. 2, March 1960, pp 265-294. Analog speakerphones generally use a switched-loss technique through which the energy of the voice signals in both a transmit and a receive direction are sensed and a switching decision made based upon that information. The voice signal having the highest energy level in a first direction will be given a clear talking path and the voice signal in the opposite direction will be attenuated by having loss switched into its talking path. If voice signals are not present in either the transmit direction or the receive direction, the speakerphone goes to an "at rest" mode which provides the clear talking path to voice signals in a receive direction favoring speech from a distance speaker. In some modern analog speakerphones, if voice signals are not present in either the transmit direction or the receive direction, the speakerphone goes to an idle mode where the loss in each direction is set to a mid-range level to allow the direction wherein voice signals first appear to quickly obtain the clear talking path.
Most high-end analog speakerphones also have a noise-guard circuit to adjust the switching levels according to the level of background noise present. Switching speed is limited by a worst-case time constant that assures that any speech energy in the room has time to dissipate. This limitation is necessary to prevent "self switching", a condition where room echoes are falsely detected as near-end speech. A disadvantage of this type of speakerphone is that no allowance is made for a room that has good acoustics, i.e., low echo energy return and short duration echoes.
With the recent availability of high-performance microcomputers for executing innovative programs, the disadvantages typically associated with the above type of speakerphone now may be overcome in a cost effective way. For example, near full to full duplex operation is available in an adaptive speakerphone such as described herein when certain conditions are satisfied. To compensate for the inherent gain between a loudspeaker in the speakerphone and a microphone also in the speakerphone, a certain amount of loss is inserted in the speakerphone circuitry to maintain stability. The amount of this loss depends upon the amount of hybrid return, the amount of acoustic return and the volume level setting. The adaptive speakerphone described herein determines these conditions and inserts the amount of switched loss necessary to maintain stability. In operation, therefore, if the adaptive speakerphone is attached to a digital telephone line and located in a good acoustic environment, the total amount of switched loss required can be significantly less than worst case. Also the decrease in switched loss while on a good analog line and in a good acoustic environment provides the user with noticeably more transparent performance.
When an adaptive speakerphone is operated in a near full or full duplex mode, speech frm the far-end party emanating from the loudspeaker is coupled back into the microphone and back through the telephone line to the far end. Because of the proximity of the loudspeaker to the microphone, the speech level at the microphone resulting from speech at the loudspeaker is typically much greater than that produced by the near-end party. This signal provided by the microphone to the far-end tends to get larger as the speakerphone approaches full duplex operation. If conversing on a handset, the far-end party perceives his or her sidetone level to be too loud and reverberant. If conversing on an adaptive speakerphone at the far-end, possible instability could result depending upon the safeguards being employed in the far-end speakerphone to guard against such an occurrence. Alternatively, limited improvement in the amount of reduced switched loss in the far-end speakerphone would be permitted because of its perceived large hybrid return value.